The most diverse methods for determining at least one physical parameter by means of a sensor unit are known from the prior art. These measurement methods frequently have in common a dependence on temperature, which is explained below with reference to inductive measuring systems, even though these statements may also apply to any other principles of measurement as desired, such as in particular capacitive or resistive measuring systems, or indeed optical or chemical measuring systems, provided the physical measured value is converted to an electrical value.
Inductive methods for detecting an object having an inductive effect have long been known. Examples of products extend from industrial proximity sensors, to position encoders, right through to treasure detecting devices. These sensors are typically provided with a coil system in which an electromagnetic field is periodically emitted and the influence on the field by the object having an inductive effect (target) is measured.
Preferably, inductive systems are used in which the transmitting coil and the receiving coil are magnetically uncoupled. In the absence of the target, the emitted magnetic field has no effect in the receiving coil. A metal object close to the transmitting field changes the magnetic lines of force or the induced eddy currents. As the eddy currents decay, a field is generated in the object and can in turn be received by the receiving coil. The received currents are naturally very small and have to be amplified for further evaluation. Depending on the form taken by the sensor system, this amplification may be more than a factor of 1 000. A problem that occurs here in practice is the effect of temperature on the analogue chips in the receive path.
The prior art, such as DE-A 10 2012 019 329, thus also discloses systems in which the received signal is controlled to give zero even when there is an influence from metal. For this purpose, for example the received signal produced in a receiving coil is controlled to give substantially zero by a compensation value that is generated in analogue manner, for example in the form of a periodically triggered voltage. The output signal of the sensor unit is reduced substantially to the fundamental wave of the excitation and if necessary the output signal is controlled to give zero in the entire phase space by means of a pulse width modulation. However, the output signal is not evaluated but rather the control signal in order to determine therefrom changes in the detection region, for example the approach of an object. The analogue signals used, by means of which the received signal of the coil system is entirely or partly compensated to zero, are not an optimum point. Generating signals that are controlled in analogue manner requires semiconductor components which are themselves relatively temperature-critical, with the result that a great deal of complexity is needed for precise measurement that is independent of temperature.
German patent application DE 10 2013 226 887, which is older but published later, discloses the fact the signal determined in the receive path at the same time contains information on the shape and/or composition of the target or the size of the surface area of the target in the detection region of the coil system. Here, the movement or position of the target are determinable by the change in shape and/or the change in composition, independently of the distance from the target to the coil system. If this information is processed by vector analysis, the desired further information can consequently be deduced in a differentiated manner. For this purpose, continuous control takes place in a closed-loop control circuit, such that the received signal is always controlled to give a zero value. Thus, a target in the detection region of the transmitting/receiving coil system always results in a change in the control signals. Thus, a deviation from the compensated condition, as an item of information relating to the target, is inherent in the control signal. If a target that has a shape or composition that is variable in a direction of measurement is used, and if, once the control variables have been appropriately taken into account, the deviation thereof from a compensated output condition is applied to a four-quadrant presentation, it can be established that the angle that an imaginary vector makes with the horizontal axis of the coordinate system is a measure of the movement of the target in the direction of measurement, while the magnitude of the imaginary vector is a measure of the distance of the target normal to the direction of measurement. Thus, the position of the target in the direction of measurement can be inferred separately from the distance of the target normal to the direction of measurement, or, depending on the target, the composition and the shape of the target can be inferred.
U.S. Pat. No. 5,729,143 A discloses an inductive metal detector in which, in a calibration phase and using a pulse width and phase modulation, undesired received signals are canceled out. Using the adjustment that is performed in this way, measurement is then performed continuously, wherein the output signals of the receiving coil for measurement are evaluated. A comparable procedure is also performed in the case of a metal detector according to U.S. Pat. No. 4,030,026 A in order to exclude mineral soils or other background conditions from the measurements.